We do learn 2 foreign languages in France during school (English + German/Spanish/Italian/whatever) so I'm not really sure why it would be a surprise to hear a french speak another language.
They might suck at it though.
I think it's because of a stereotype in America/Australia etc that the French are very protective of their language (see http://en.wikipedia.org/wiki/Language_policy_in_France ) and would rather people learned French than "lower themselves" to using English. When I was in Paris in the mid 90s there was some sense of that.
It's probably changed now - while I haven't been to France lately, in other parts of Europe when I try to practice the local language, people under 30 always tell me "Please, speak English, we understand and it is easier for both of us."
French people "learn" german and english but are terrible at it even after learning it for 10 years.
Scandinavian people learn english and are actually fluent with a good pronunciation after a few years.